UK Property Law
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The legal position is that it is not for him to paint your side of the wall if it is on your property - you should invite hi to make good or there is a potential claim against him for trespass and damages - basically the cost to make good. This can be resolved through the small claims court. I would suggest a letter to attempt to resolve. Please rate positive.
If the wall is on your land as you state then you are entitled to paint the wall in the way in which you choose. happy to discuss - please rate positive
I hope that this helps - please rate positive - thanks